1. Field of the Invention
The present invention relates to an optical transmitter, in particular, the invention relates to an optical transmitter implementing a wavelength tunable laser diode (t-LD) capable of outputting light with a narrowed linewidth.
2. Background Arts
Various prior documents have reported to narrow the linewidth of the light emitted from an LD, where most of the prior techniques disclosed in the documents has fed the output light detected through an optical filter, whose transmittance shows wavelength dependence, back to a current injected into the LD. However, such a conventional technique is hard to be applied to an optical transmitter implemented within the optical coherent system.
The optical coherent system, in particular, the digital coherent system requests the optical transmitter to provide light with the spectral linewidth less than 300 kHz. Also, the digital coherent system inevitably requires the function for the optical source to tune the emission wavelength thereof. Accordingly, an LD implemented therein provides a structure including a gain section, a phase shifting section, and two Bragg reflector sections to tune the emission wavelength thereof. Each of those sections causes phase noises in the light output from the LD. The conventional technique to reduce the phase noise compensates only one of those sections. However, the compensation for the one section sometimes enhances the phase noises in the other sections, which degrades the RIN (relative intensity noise) in the output and inevitably sets the spectral linewidth of the light in an unsatisfactory performance.